EUV (Extreme Ultra-Violet) lithography is expected as a method for forming finer patterns than patterns formed by conventional exposing light (193 nm or 248 nm in wavelength) on wafers, with the use of light (X rays) of a very short wavelength around 13.5 nm.
In an EUV exposure device, all the optical components of the optical system of illumination for EUV masks and the optical system for projections onto wafers are formed with the use of reflecting mirrors, because of the properties of EUV light with short wavelengths. Therefore, scattered light generated from EUV light is observed, depending on the flatness of the surface of each reflecting mirror. The scattered light is projected as stray light in a different form from the patterns to be exposed on wafers. Such stray light is called “flare”.
To reduce the impact of flare, device manufactures have improved the flatness of each reflecting mirror in exposure devices. At the same time, to obtain a desired pattern size in a situation where flare exists, a flare correction to change the shape of the exposure pattern is effectively performed.
In a flare correction method, a pattern to be exposed is divided into meshes, and the pattern shape and size are corrected, with the flare amount in the patterns of each of the meshes being regarded as constant (see James Word, et al., “Full Chip Model Based Correction of Flare-Induced Linewidth Variation”, Proceedings of SPIE, Vol. 5567, 2004, p.p. 700-710, for example). According to this method, the mesh size is approximately 1/10 of the spread diameter of the flare, and the flare amount is determined based on the pattern area ratio (the pattern density) in the meshes. A pattern correction is then performed so that each of the patterns in each mesh exposed and formed when the determined flare exists has the desired shape and size. With the spread diameter of the flare being taken into consideration, the accuracy of flare amount estimation is increased through a convolution calculation between the pattern area ratio in each mesh and a Gauss function representing the spread diameter of the flare.
However, the flare generated from reflecting mirrors is scattered over a region of several millimeters to several tens of millimeters. Therefore, the convolution integration between the pattern area density and a flare point spread function involves a very large amount of calculation, resulting in problems such as increases in computer costs and prolongations of calculation time.
Furthermore, the spread of flare is not always represented by Gauss functions. If fitting with the sum of Gauss functions is performed for a calculation, calculation errors become larger, and the accuracy of flare amount estimation becomes lower. If the accuracy of flare amount estimation becomes lower, precise patterns are not formed for semiconductor devices manufactured with the use of a mask pattern corrected to eliminate the impact of flare. As a result, problems such as poorer performance and a lower production yield are caused.